The present invention pertains to electrical devices and, more particularly, to sensors and sensing methods suitable for three-phase, gas insulated devices, e.g., switchgears and transmission lines.
Electrical power transmission/distribution systems typically provide for a switch to be interposed between a transformer and a line in the system. This switch (commonly referred to as “switchgear”) provides a mechanism whereby the transformer can be disconnected from the rest of the system in response to, e.g., a power surge or other type of system occurrence. Recently, switchgear has been insulated using an insulating gas, e.g., SF6, since the insulating properties of such gases enable the overall size of the switchgear to be reduced significantly. Initially, gas insulated switchgear was proposed as a single phase solution, i.e., each of the three phases had a separately packaged switch associated therewith. More recently, to further reduce the size associated with the switchgear, three-phase, gas insulated switchgear has been proposed, an example of which is described in U.S. Published Patent Application No. 2003/0178891, the disclosure of which is incorporated here by reference.
FIG. 1 illustrates a conventional, single-phase, gas insulated switchgear (GIS) device 1. Therein, the GIS device includes bus bars 2 for each pole, disconnection switches 3, grounding switches 4, 5 and circuit breaker 6. Each of the three enclosures 11 associated with a single phase of the GIS device 1, is filled with an insulating gas, e.g. SF6 gas. The enclosures also include mechanical parts of the switches, e.g., contacts, drive mechanisms, etc., associated with the GIS device I and the electrical conductor (not shown in FIG. 1) which conducts its respective phase's current through the switchgear 1. This configuration is referred to herein as “single-phase, gas insulated switchgear” because current associated with each of the three electrical phases is conveyed via a conductor which is enclosed within its own, gas insulated ground enclosure 11.
By way of contrast, three-phase gas insulated switchgear refers to switchgear wherein three conductors (one for each phase) are conveyed through a single, insulating gas-filled enclosure, an example of which is illustrated in FIG. 2. Therein, the three separate enclosures 11 shown in FIG. 1 are replaced by a single enclosure 20 having two endcaps 22 and three electrical conductors 24, 26, and 28 (one for each phase) extending between the two endcaps 22. In practice, an enclosing cover (not shown) would surround the three conductors and completely enclose the region between the two endcaps 22, but this has been removed in FIG. 2 to illustrate the orientation of the conductors.
In electrical switchgear, it can be useful to measure the current flowing through each conductor and the voltage between two electrodes, e.g. a high voltage conductor and ground. Various types of sensors have been used in single-phase, gas insulated switchgear to perform these functions. For example, current and voltage transformers or optical current and voltage sensors have been placed individually proximate to each enclosure 11 to monitor that phase's current and/or voltage. However, the introduction of three-phase GIS devices complicates the sensing environment since all three phases are carried within a single enclosure.
Accordingly, it would be desirable to provide sensors and sensing methods which are able to accurately detect current and/or voltage in three-phase GIS devices.